A fuel vapor treatment system is used for preventing fuel vapor produced in a fuel tank from being dissipated into the atmosphere and introduces the fuel vapor in the fuel tank into a canister accommodating an adsorbent to adsorb the fuel vapor temporarily by the adsorbent. The fuel vapor adsorbed by the adsorbent is desorbed by negative pressure produced in an intake pike when an internal combustion engine is operated and is purged into the intake pipe of the internal combustion engine through a purge passage. When the fuel vapor is desorbed from the adsorbent in this manner, the adsorbing capacity of the adsorbent is recovered.
When the fuel vapor is purged, the flow rate of an air-fuel mixture containing the fuel vapor is adjusted by a purge control valve provided in the purge passage. However, to adjust the amount of fuel vapor actually purged into the intake pipe to a suitable air-fuel ratio by the purge control valve, it is important to measure the concentration of the fuel vapor in the air-fuel mixture flowing through the purge passage with high accuracy.
JP-2004-116303A shows a fuel vapor treatment apparatus having a throttle in a purge passage to calculate the fuel vapor concentration based on a differential pressure between upstream and downstream of the throttle. In this apparatus, the fuel vapor concentration is calculated based on a basic differential pressure in which the fuel vapor concentration is 0%. Since it is hard to practically create the condition in which the fuel vapor concentration is 0%, the basic differential pressure is pre-calculated and is stored in an ECU. However, the pre-calculated basic differential pressure may have errors in a case that the pressure sensor is deteriorated or a pressure loss in the treatment system is varied with age. The differential pressure in the throttle depends on density of fluid flowing through the throttle. When the ambient pressure or ambient temperature is varied, the density is also varied, which may cause errors.